Current source circuit and differential amplifier

ABSTRACT

A current source block and a negative resistance generation block are connected in parallel. The negative resistance generation block generates a negative resistance in response to the minute variations of an output voltage. Thus the output resistance of a current source circuit is given by the combined resistance of the negative resistance and the resistance of a resistor in the current source block connected in parallel. The resistance of the resistor in the current source block and the negative resistance are controlled to be substantially the same to thereby increase the output resistance of the current source circuit. The current source circuit serves to increase an output resistance when viewed from an differential output terminal. As a result, use of this current source circuit realizes a differential amplifier providing a high gain.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a current source circuit and adifferential amplifier, and more particularly to a current sourcecircuit realizing increase of an output resistance and a differentialamplifier using this current source circuit.

2. Description of the Background Art

A current source providing a high output resistance (rds) is ideallysuited in a semiconductor integrated circuit. However, a recent trendfor downsizing process makes a shorter channel length of an MOStransistor. The output resistance of a current source including an MOStransistor is reduced accordingly by short channel effect.

In a differential amplifier using such a current source, the outputresistance of the differential amplifier is reduced when viewed from adifferential output terminal providing a differential signal. Thedifferential amplifier provides a gain that is in proportion to theoutput resistance of the differential amplifier when viewed from thedifferential output terminal. Thus the reduced output resistance of thedifferential amplifier results in reduced gain of the differentialamplifier.

The document named below tries to increase the gain of a differentialamplifier using cascode connection:

“Design of Analog CMOS Integrated Circuits”, Behzad Razavi, MacGraw-HilSeries in Electrical and Computer Engineering, pp. 296-299 (FIGS. 9.6(b) and 9.8 (b)).

However, the differential amplifier using cascode connection fails toprevent the reduction of an output resistance caused by short channeleffect, thus making it difficult to increase the gain of thedifferential amplifier.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a circuitconfiguration that easily provides a current source with a high outputresistance even through the output resistance of an MOS transistor isreduced due to short channel effect.

According to a first aspect of the present invention, a current sourcecircuit includes an output terminal, a current source including aninternal resistor and connected to the output terminal, and a negativeresistance generation block generating a negative resistance andconnected in parallel to the current source.

The negative resistance generation block generating a negativeresistance is connected in parallel to the current source including aninternal resistor. Thus the output resistance of the current sourcecircuit when viewed from the output terminal is given by the combinedresistance of the internal resistor of the current source and thenegative resistance generation block connected in parallel to thecurrent source. When the resistance in the current source is reduced dueto short channel effect, the resistance of the internal resistor and thenegative resistance are controlled to be substantially the same. As aresult, the output resistance of the current source circuit can beincreased.

According to a second aspect of the present invention, a differentialamplifier includes a differential input terminal, a differential outputterminal providing a differential output signal formed by amplifying adifferential input signal sent from the differential input terminal, anda negative resistance generation block generating a negative resistanceand connected to the differential output terminal.

The differential amplifier includes the negative resistance generationblock generating a negative resistance and connected to the differentialoutput terminal. Thus the output resistance when viewed from thedifferential output terminal is given by the combined resistance of theresistor and the negative resistance generation block of thedifferential amplifier connected in parallel to the resistor. Thedifferential amplifier provides a gain that is in proportion to theoutput resistance when viewed from the differential output terminal.Thus a gain can be increased by controlling the output resistance andnegative resistance of the differential amplifier to be substantiallythe same.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing the configuration of a currentsource circuit according to a first preferred embodiment of the presentinvention;

FIG. 2 is a small-signal equivalent circuit of the current sourcecircuit according to the first preferred embodiment;

FIG. 3 is a circuit diagram showing the configuration of a differentialamplifier according to a second preferred embodiment of the presentinvention;

FIG. 4 is a small-signal equivalent circuit of the differentialamplifier according to the second preferred embodiment;

FIGS. 5 and 6 are circuit diagrams each showing the configuration of adifferential amplifier according to a third preferred embodiment of thepresent invention;

FIGS. 7 and 8 are circuit diagrams each showing the configuration of adifferential amplifier according to a fourth preferred embodiment of thepresent invention;

FIG. 9 is a circuit diagram showing the configuration of a differentialamplifier according to a fifth preferred embodiment of the presentinvention;

FIG. 10 is a circuit diagram showing the configuration of a differentialamplifier according to a sixth preferred embodiment of the presentinvention; and

FIGS. 11 and 12 are circuit diagrams each showing the configuration of adifferential amplifier according to a seventh preferred embodiment ofthe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Preferred Embodiment

FIG. 1 is a circuit diagram showing the configuration of a currentsource circuit 14 according to a first preferred embodiment of thepresent invention. The current source circuit 14 has a parallelconnection of a current source block (current source) 11 and a negativeresistance generation block 12.

The negative resistance generation block 12 is configured as follows.The input of an amplifier A1 (inverting amplifier) providing a voltagegain of −1 is connected to an output terminal O1 (first differentialoutput terminal). The output of the amplifier A1 is connected to avoltage-controlled terminal MG1 of an MOS transistor (voltage-controlledcurrent source) M1. The MOS transistor M1 has a terminal MD1 (currentinput terminal) connected to the output terminal O1, and a terminal MS1(current output terminal) connected at a node a to one end of a resistor(internal resistor) Rne1 (in the following, the resistance of theresistor Rne1 will also be identified as Rne1). Another end of theresistor Rne1 is grounded. A constant current source I21 is connected inparallel to the resistor Rne1. The resistor Rne1 and the constantcurrent source I21 form a second current source.

The current source block 11 includes a resistor (internal resistor) Rcu(in the following, the resistance of the resistor Rcu will also beidentified as Rcu), and a constant current source I11. The resistor Rcuhas one end connected to the terminal MD 1 of the MOS transistor M1, andanother end that is grounded. The resistor Rcu and the constant currentsource I11 are connected in parallel.

Next, it will be discussed with reference to FIG. 2 how the currentsource circuit 14 having the foregoing configuration operates. FIG. 2 isa small-signal equivalent circuit of the current source circuit 14according to the first preferred embodiment. The output terminal O1 isconnected to the input of the amplifier A1. The output of the amplifierA1 is connected to the gate terminal MG1 of the MOS transistor M1.

The source terminal MS1 of the MOS transistor M1 is connected to one endof the resistor Rne1. Another end of the resistor Rne1 is grounded. Theterminal MD1 of the MOS transistor M1 is connected to the outputterminal O1 and to one end of the resistor Rcu. Another end of theresistor Rcu is grounded. In FIG. 2, the MOS transistor M1 isrepresented as an equivalent circuit including a voltage-controlledcurrent source 21 and a resistor R (whose resistance will also beidentified as R) connected in parallel thereto. The resistor Rcu in thecurrent source block 11 has one end connected to the output terminal O1and to the drain terminal MD1 of the MOS transistor M1, and another endthat is grounded.

First, it will be discussed how the negative resistance generation block12 operates. A voltage Vout1 (output signal) is applied as a smallsignal to the output terminal O1 to cause a flow of a current IF in thenegative resistance generation block 12. At this time, an outputresistance Rout of the negative resistance generation block 12 is givenby:Rout=Vout 1/IF  (1)The current IF caused to flow by the application of the voltage Vout1thereafter flows into the resistor Rne. Thus the current IF is expressedas:IF=Va/Rne  (2)where Va is a voltage at the node a.

The input of the voltage Vout1 causes the amplifier A1 to output avoltage −Vout1 as an inverted output signal. This causes the flow of acurrent ID in the voltage-controlled current source 21 given by:ID=gm·(−Vout 1−Va)  (3)where gm represents a mutual conductance of the voltage-controlledcurrent source 21.

A current expressed as (IF−ID) flows through the internal resistor R.The voltage at the node a is thus expressed as:Va=Vout 1−R·(IF−ID)  (4)

Expressions (2) and (3) are substituted for IF and ID in expression (4)respectively, whereby the voltage Va is expressed as:Va=(Rne·(1−gm·R)/(R+(1+gm ·R)·Rne))·Vout

When (gm·R>>1) and (gm·Rne >>1), the voltage Va is expressed as:Va=(−gm·Rne)/(1+gm·Rne)·Vout

When (gm R>>1) again, the voltage Va is also expressed as follows:Va≈−Vout  (5)

Using expressions (1), (2) and (5), the output resistance Rout of thenegative resistance generation block 12 is finally given by:Rout=Vout 1/IF≈Vout 1/(−Vout 1/Rne)=−Rne

As seen from the foregoing, the output resistance Rout of the negativeresistance generation block 12 is expressed as −Rne. That is, thenegative resistance generation block 12 serves to generate the negativeresistance −Rne when viewed in an equivalent circuit. The negativeresistance generation block 12 is connected in parallel to the resistorRcu in the current source block 11. Then the combined resistance of thenegative resistance generation block 12 and the resistor Rcu in thecurrent source block 11 is given by:Rout=Rcu//−Rne=Rcu·Rne/(Rne−Rcu)  (6)where (Rcu//−Rne) represents the combined resistance of the resistancesRcu and −Rne1 connected in parallel. If the resistance Rne1 iscontrolled to be substantially the same as the resistance Rcu inexpression (6), the output resistance Rout of the current source circuit14 can be increased.

When the resistances Rne1 and Rcu are reduced by short channel effect,the resistances Rne1 and Rcu are controlled to be substantially thesame. As a result, the output resistance Rout of the current sourcecircuit 14 can be increased.

Second Preferred Embodiment

FIG. 3 is a circuit diagram showing the configuration of a differentialamplifier according to a second preferred embodiment of the presentinvention. The differential amplifier of the second preferred embodimentemploys the current source circuit 14 of the first preferred embodiment.In FIG. 3, respective resistors R11 and R12 of loads R1 and R2, andrespective resistors Rin1 and Rin2 of voltage-controlled current sourcesT1 and T2 are represented by dotted lines. In the following, therespective resistances of the resistors R11, R12, Rin1 and Rin2 will beidentified by the same reference numerals.

The load R1 has one end connected to a power source voltage VDD, andanother end connected to the output terminal (first differential outputterminal) O1. The voltage-controlled current source T1 has a terminal D1connected to the output terminal O1, and a terminal S1 connected to oneend of a constant current source It. Another end of the constant currentsource It is grounded. The voltage-controlled current source T1 also hasa voltage-controlled terminal (differential input terminal) G1 connectedto one end of a voltage source Vin1 (whose voltage will also beidentified as Vin1). The input voltage Vin1 (differential input signal)is applied to the voltage-controlled terminal G1. Another end of thevoltage source Vin1 is grounded. The voltage-controlled current sourceT1 corresponds to the current source block 11 shown in FIG. 1.

The MOS transistor M1 has the terminal MD1 connected to the outputterminal O1, and the terminal MS1 connected to one end of the resistorRne1. Another end of the resistor Rne1 is grounded. The resistor Rne1 isconnected in parallel to the constant current source I21.

The gate terminal MG1 of the MOS transistor M1 is connected to an outputterminal (second differential output terminal) O2. An MOS transistor M2has a terminal MD2 connected to the output terminal O2, and a terminalMS2 connected to one end of a constant current source I22. Another endof the constant current source I22 is grounded. A resistor Rne2 isconnected in parallel to the constant current source I22.

The load R2 has one end connected to the output terminal O2, and anotherend connected to the power source voltage VDD. The voltage-controlledcurrent source T2 has a terminal D2 connected to the output terminal O2,and a terminal S2 connected to one end of the constant current sourceIt. The voltage-controlled current source T2 also has avoltage-controlled terminal G2 connected to one end of a voltage sourceVin2 (whose voltage will also be identified as Vin2). Another end of thevoltage source Vin2 is grounded. The voltage sources Vin1 and Vin2 eachserve to generate a differential input signal.

Next, it will be discussed how the differential amplifier having theforegoing configuration operates. In FIG. 3, the left half and the righthalf of the differential amplifier have the same circuit configuration.When the voltage source Vin2 outputs a signal having the same amplitudeas and opposite polarity to a signal given from the voltage source Vin1,an output voltage Vout2 given from the output terminal O2 (inverteddifferential output signal) thus has the same level as and oppositepolarity to the output voltage Vout1.

The voltage-controlled terminal MG1 of the MOS transistor M1 receivesthe output voltage Vout2 (=−Vout1) that is the same as a signal sentfrom the amplifier A1 (in FIG. 1). Thus the MOS transistor M1, resistorRne1 and constant current source I21 form the negative resistancegeneration block 12 discussed in the first preferred embodiment withreference to FIG. 1.

FIG. 4 shows a small-signal equivalent circuit of the left half of thecircuit configuration of the differential amplifier according to thesecond preferred embodiment. The resistor Rin1 of the voltage-controlledcurrent source T1 is connected in parallel to the resistor R11 and theresistor having the resistance −Rne1. The resistor Rin1 has one endgrounded and another end connected to the output terminal O1.

While one end of the load R1 is connected to the power source voltageVDD in FIG. 3, it is substantially grounded in the small-signalequivalent circuit. Thus in FIG. 4, the resistor R11 of the load R1 isshown to be connected in parallel to the resistor Rin1 of thevoltage-controlled current source T1. Further, the negative resistancegeneration block 12 is represented as a parallel connection of theresistor Rin1 and the resistor with the resistance −Rne1.

With reference to FIG. 4, the application of the input voltage Vin1defines the output voltage Vout1 (differential output signal) asfollows:Vout 1=−gm1·Vin 1·(Rin 1//R 11//−Rne 1)where gm1 represents a mutual conductance of the voltage-controlledcurrent source T1.

A gain A2 of the differential amplifier is finally given by:A2=Vout 1/Vin 1=−gm1·(Rin 1//R 11//−Rne 1)The configuration on the right side designated by different referencenumerals produces the same result as that given by the left side, andhence will not be discussed.

A gain A0 obtained in a background-art differential amplifier is givenby:A 0=−gm 1·(Rin 1//R11)The combined resistance Rin1//Rl1 of the resistors Rin1 and R11 will bereferred to as an output resistance of the background-art amplifier. Thegain A0 is reduced by the decrease of the resistance of the resistorRin1 of the voltage-controlled current source T1.

The second preferred embodiment employs the current source circuit 14 inwhich the voltage-controlled current source T1 and the negativeresistance generation block 12 are connected in parallel. Then the gainA2 is given by:A 2=−gm 1·(Rin 1//R 11//−Rne 1)

When the resistance of the resistor Rin1 of the voltage-controlledcurrent source T1 is reduced, the output resistance (Rin1//R11) of thebackground-art differential amplifier and the value of the negativeresistance −Rne1 are controlled to be substantially the same. As aresult, the gain A2 can be increased.

Third Preferred Embodiment

FIG. 5 is a circuit diagram showing the configuration of a differentialamplifier according to a third preferred embodiment of the presentinvention. In the third preferred embodiment, the differential amplifierof the second preferred embodiment is more specifically configured usingMOS transistors.

A PMOS transistor Mp1 has a source connected to the power source voltageVDD. The substrate terminal of the PMOS transistor Mp1 is also connectedto the power source voltage VDD. The PMOS transistor Mp1 has a drainconnected to the output terminal O1. An NMOS transistor Mn1 has a drainconnected to the output terminal O1, a source connected to one end ofthe constant current source It, and a substrate terminal connected tothe source.

Another end of the constant current source It is grounded. The gate ofthe NMOS transistor Mn1 is connected to one end of the voltage sourceVin1. Another end of the voltage source Vin1 is grounded.

An NMOS transistor Mn3 has a drain connected to the output terminal O1,and a source and a substrate terminal connected to the drain of an NMOStransistor Mn5. The NMOS transistor Mn5 has a source that is grounded,and a gate subjected to the application of a voltage Vb1.

The NMOS transistor Mn3 has a gate connected to the output terminal O2.An NMOS transistor Mn4 has a drain connected to the output terminal O2.The NMOS transistor Mn4 has a source and a substrate terminal connectedto the drain of an NMOS transistor Mn6. The NMOS transistor Mn6 has asource that is grounded, and a gate subjected to the application of thevoltage Vb1.

A PMOS transistor Mp2 has a drain connected to the output terminal O2,and a source connected to the power source voltage VDD. An NMOStransistor Mn2 has a drain connected to the output terminal O2, a sourceconnected to one end of the constant current source It, and a gateconnected to one end of the voltage source Vin2. Another end of thevoltage source Vin2 is grounded.

The voltages Vb1 and Vb2 are controlled such that the NMOS transistorsMn5 and Mn6, and the PMOS transistors Mp1 and Mp2 operate in asaturation region.

Next, it will be discussed how the differential amplifier having theforegoing configuration operates.

In the third preferred embodiment, the NMOS transistors Mn3 and MN5 formthe negative resistance generation block 12. The NMOS transistor Mn3corresponds to the MOS transistor M1 (FIG. 3). The NMOS transistor(current source) Mn5 corresponds to the constant current source I21 andthe resistor Rne1 connected in parallel to the constant current sourceI21 (FIG. 3). The voltage Vb1 is applied to the NMOS transistor Mn5 suchthat the NMOS transistor Mn5 operates in a saturation region. Thenidentifying the output resistance of the NMOS transistor Mn5 as m5, theoutput resistance m5 corresponds to the resistance Rne1, and thenegative resistance generation block 12 generates a negative resistance−rn5.

The PMOS transistor Mp1 corresponds to the load R1, and the NMOStransistor Mn1 corresponds to the voltage-controlled current source T1.Then identifying the respective output resistances of the PMOStransistor Mp1 and NMOS transistor Mn1 as rp1 and rn1, a gain A3 of thedifferential amplifier of the third preferred embodiment is given by:A 3=−gmn 1·(rn 1//rp 1//−rn 5)where gmn1 is a mutual conductance of the NMOS transistor Mn1.

In the configuration discussed so far, the combined resistance of therespective output resistances rn1 and rp1 of the NMOS transistor Mn1 andPMOS transistor Mp1 (rn1//rp1) (the combined resistance obtained in abackground-art differential amplifier), and the output resistance rn5 ofthe NMOS transistor Mn5 are controlled to be substantially the same. Asa result, the gain A3 can be increased.

In the third preferred embodiment, the substrate terminals of the NMOStransistors Mn1, Mn2, Mn3 and Mn4 are connected to the respectivesources. These substrate terminals may alternatively be grounded asshown in FIG. 6, in which case the NMOS transistors Mn1 through Mn4 aresubjected to substrate bias effect. This configuration also provides theoutput resistance (rn1//rp1//−m5) when viewed from the output terminalO1.

A gain A3H of the differential amplifier shown in FIG. 6 is given by:A 3 H=−gmn 1·(rn 1//rp 1//−rn 5)The combined resistance obtained in the background-art differentialamplifier (rn1//rp1) and the output resistance rn5 of the NMOStransistor Mn5 are controlled to be substantially the same. As a result,the gain A3H can be increased. The substrate terminals of the NMOStransistors Mn3 and Mn4 may be connected to the respective sources, inwhich case the same effect can be obtained.

The foregoing description has been made with reference to the left halfof the circuit configuration. The right half of the circuit has the sameconfiguration as that of the left half, and produces the same effect.Further, even the absence of the constant current source It alsoprovides the same effect.

Fourth Preferred Embodiment

FIG. 7 is a circuit diagram of a differential amplifier according to afourth preferred embodiment of the present invention. In the fourthpreferred embodiment, the differential amplifier of the second preferredembodiment is more specifically configured using MOS transistorsopposite in polarity to those of the third preferred embodiment.

The NMOS transistor Mn1 has a drain connected to the output terminal O1,and a substrate terminal connected to a source that is grounded. Thegate of the NMOS transistor Mn1 is subjected to the application of thevoltage Vb2 such that the NMOS transistor Mn1 operates in a saturationregion.

The PMOS transistor Mp1 has a drain connected to the output terminal O1,a gate connected to the voltage source Vin1 for receiving the inputvoltage Vin1, a substrate terminal connected to a source that isconnected through the constant current source It to the power sourcevoltage VDD. The source of the PMOS transistor Mp1 is also connected tothe source of the PMOS transistor Mp2. The PMOS transistor Mp2 has adrain connected to the output terminal O2, a gate connected to thevoltage source Vin2 for receiving the input voltage Vin2, and asubstrate terminal connected to the source.

The NMOS transistor Mn2 has a drain connected to the output terminal O2,a source that is grounded, and a gate subjected to the application ofthe prescribed voltage Vb2 such that the NMOS transistor Mn2 operates ina saturation region. The substrate terminal of the NMOS transistor Mn2is connected to the source. A PMOS transistor Mp5 has a drain connectedto the drain of the NMOS transistor Mn1 and to the gate of a PMOStransistor Mp6.

The PMOS transistor Mp5 has a substrate terminal connected to a sourcethat is connected to the drain of the PMOS transistor Mp3. The gate ofthe PMOS transistor Mp5 is connected to the output terminal O2 and tothe drain of the PMOS transistor Mp6. The substrate terminal of the PMOStransistor Mp3 is connected to a source that is connected to the powersource voltage VDD. The gate of a PMOS transistor Mp3 is subjected tothe application of the voltage Vb1 of a prescribed level such that thePMOS transistor Mp3 operates in a saturation region.

The drain of the PMOS transistor Mp6 is connected to the gate of thePMOS transistor Mp5, to the output terminal O2 and to the drain of theNMOS transistor Mn2. The substrate terminal of the PMOS transistor Mp6is connected to a source that is connected to the drain of a PMOStransistor Mp4. The gate of the PMOS transistor Mp6 is connected to thedrain of the PMOS transistor Mp5.

The substrate terminal of the PMOS transistor Mp4 is connected to asource that is connected to the power source voltage VDD. The gate ofthe PMOS transistor Mp4 is subjected to the application of theprescribed voltage Vb1 such that the PMOS transistor Mp4 operates in asaturation region.

Next, it will be discussed how the differential amplifier having theforegoing configuration operates. In the fourth preferred embodiment,the PMOS transistors Mp3 and Mp5 form the negative resistance generationblock 12. The PMOS transistor Mp5 corresponds to the MOS transistor M1(FIG. 3). The voltage Vb1 is applied to the PMOS transistor Mp3 suchthat the PMOS transistor Mp3 operates in a saturation region. The PMOStransistor Mp3 corresponds to the constant current source I21 and theresistor Rne1 connected in parallel to the constant current source I21(FIG. 3).

Identifying the output resistance of the PMOS transistor Mp3 as rp3, theoutput resistance rp3 corresponds to the resistance Rne1 and thenegative resistance generation block 12 generates a negative resistance−rp3. The NMOS transistor Mn1 corresponds to the load R1, and the PMOStransistor Mp1 corresponds to the voltage-controlled current source T1.

Then identifying the respective output resistances of the NMOStransistor Mn1 and PMOS transistor Mp1 as rn1 and rp1, a gain A4 of thedifferential amplifier of the fourth preferred embodiment is given by:A 4=−gmn 1·(rn 1//rp 1//−rp 3)where gmn1 is a mutual conductance of the PMOS transistor Mp1.

In the configuration discussed so far, the combined resistance of theoutput resistance m1 of the NMOS transistor Mn1 and the outputresistance rp1 of the PMOS transistor Mp1 (rn1//rp1) (the combinedresistance obtained in a background-art differential amplifier), and theoutput resistance rp3 of the PMOS transistor Mp3 are controlled to besubstantially the same. As a result, the gain A4 can be increased.

In the fourth preferred embodiment, the substrate terminals of the PMOStransistors Mp1 and Mp2 are connected to the respective sources. Thesesubstrate terminals may alternatively be connected to the power sourcevoltage VDD as shown in FIG. 8, in which case the PMOS transistors Mp1and Mp2 are subjected to substrate bias effect. This configuration alsoprovides the output resistance (rn1//rp1//−rp3) when viewed from theoutput terminal O1.

A gain A4H of the differential amplifier shown in FIG. 8 is given by:A 4 H=−gmn 1·(rn 1//rp 1//−rp 3)The combined resistance obtained in the background-art differentialamplifier (rn1//rp1) and the output resistance rp3 of the PMOStransistor Mp3 are controlled to be substantially the same. As a result,the gain A4H can be increased.

The foregoing description has been made with reference to the left halfof the circuit configuration. The right half of the circuit has the sameconfiguration as that of the left half, and produces the same effect.Further, even the absence of the constant current source It alsoprovides the same effect.

Fifth Preferred Embodiment

FIG. 9 is a circuit diagram showing the configuration of a differentialamplifier according to a fifth preferred embodiment of the presentinvention. In the fifth preferred embodiment, circuits 13 and 15 (secondcurrent source) are added to the negative resistance generation block 12of the second preferred embodiment. The other constituents operating inthe same as those of the second preferred embodiment are designated bythe same reference numerals, and will not be discussed.

The circuit 13 includes a constant current source I31, and a resistorR13 (whose resistance will also be identified as R13) connected inparallel to the constant current source I31. The resistor R13 has oneend connected to the power source voltage VDD, and another end connectedto the resistor Rne1. The circuit 15 on the right side also includes aconstant current source 132, and a resistor R14 connected in parallel tothe constant current source 132.

First, the output resistance of the negative resistance generation block12 will be discussed. In the small-signal equivalent circuit of thenegative resistance generation block 12, the power source voltage VDD ofthe circuit 13 is grounded with respect to alternating-currentcomponents. The constant current sources I31 and I21 do not appear inthe small-signal equivalent circuit. That is, the circuit 13 is coupledin ac-parallel to the circuit (first current source) formed by theresistor Rne1 and the constant current source I21. Then the outputresistance of the negative resistance generation block 12 is given bythe combined resistance of the resistances of the resistors R13 and Rne1(R13//Rne1).

The combined resistance (R13//Rne1) corresponds to the resistance Rne1of the second preferred embodiment due to the existence of the circuit13. Thus the negative resistance generation block 12 of the fifthpreferred embodiment is operative to generate a negative resistance−(Re13//Rne1) in the equivalent circuit.

Then a gain A5 of the differential amplifier of the fifth preferredembodiment is given by:A 5=−gm 1·(Rin 1//R 11//−Rne 1//−R 13)

The output resistance of the background-art amplifier (Rin1//R11) isgenerally given by the output resistances of MOS transistors of oppositepolarities. As an example, the voltage-controlled current source T1 isformed by an NMOS transistor and the load R1 is by a PMOS transistor. Incontrast, the negative resistance generation block 12 of the secondpreferred embodiment uses an MOS transistor in one polarity to generatea negative resistance. Thus the output resistance of the negativeresistance generation block 12 of the second preferred embodiment ishard to coincide in level with the output resistance (Rin1//Rl1) of thebackground-art amplifier.

In the fifth preferred embodiment, the negative resistance generationblock 12 has the similar configuration to that of the background-artdifferential amplifier in which MOS transistors of opposite polaritiesare employed.

As an example, the resistor R13 may be a PMOS transistor and theresistor Rne1 may be an NMOS transistor. In this case, the outputresistance is easily allowed to have a very close level to and oppositepolarity to the background-art output resistance (Rin1//R11). Thiseasily provides an increased output resistance.

Sixth Preferred Embodiment

FIG. 10 is a circuit diagram showing the configuration of a differentialamplifier according to a sixth preferred embodiment of the presentinvention. In the sixth preferred embodiment, the differential amplifierof the fifth preferred embodiment is more specifically configured usingMOS transistors. Except for the presence of the circuits 13 and 15, thedifferential amplifier of the sixth preferred embodiment has the sameconfiguration as that of the third preferred embodiment (FIG. 5). Theconstituents operating in the same manner as those of the thirdpreferred embodiment are designated by the same reference numerals, andwill not be discussed.

The configuration of the circuit 13 will be discussed first. The PMOStransistor Mp3 has a source connected to the power source voltage VDD, adrain connected to the drain of the NMOS transistor Mn5, and a substrateterminal connected to the source. A voltage Vb3 is applied to the gateof the PMOS transistor Mp3 such that the PMOS transistor Mp3 operates ina saturation region.

Then the configuration of the circuit 15 will be discussed. The PMOStransistor Mp4 has a source connected to the power source voltage VDD, adrain connected to the drain of the NMOS transistor Mn6, and a substrateterminal connected to the source. The voltage Vb3 of a prescribed levelis applied to the gate of the PMOS transistor Mp4 such that the PMOStransistor Mp4 operates in a saturation region.

Next, it will be discussed how the differential amplifier having theforegoing configuration operates. In the sixth preferred embodiment, theNMOS transistors Mn3, Mn5 and PMOS transistor Mp3 form the negativeresistance generation block 12. The NMOS transistor Mn3 corresponds tothe MOS transistor M1 (FIG. 9). The NMOS transistor Mn5 corresponds tothe constant current source I21 and the resistor Rne1 connected inparallel to the constant current source I21 (FIG. 9). The voltage Vb1 isapplied to the NMOS transistor Mn5 such that the NMOS transistor Mn5operates in a saturation region.

The PMOS transistor (second current source) Mp3 corresponds to theconstant current source I31 and the resistor R13 connected in parallelto the constant current source I31 (FIG. 9). The voltage Vb3 is appliedto the PMOS transistor Mp3 such that the PMOS transistor Mp3 operates ina saturation region. Thus the output resistance m5 of the NMOStransistor Mn5 corresponds to the resistance Rne1. The output resistancerp3 of the PMOS transistor Mp3 corresponds to the resistance R13. Thusthe negative resistance generation block 12 generates a negativeresistance (−rn5//−rp3). The PMOS transistor Mp1 corresponds to the loadR1, and the NMOS transistor Mn1 corresponds to the voltage-controlledcurrent source T1. Then identifying the respective output resistances ofthe PMOS transistor Mp1 and NMOS transistor Mn1 as rp1 and rn1, a gainA6 of the differential amplifier of the sixth preferred embodiment isgiven by:A 6=−gmn 1·(rn 1//rp 1//−rn 5//−rp 3)where gmn1 is a mutual conductance of the NMOS transistor Mn1.

In the configuration discussed so far, the combined resistance of therespective output resistances rn1 and rp1 of the NMOS transistor Mn1 andPMOS transistor Mp1 (rn1//rp1), and the value of the combined resistanceof the respective output resistances rn5 and rp3 of the NMOS transistorMn5 and PMOS transistor Mp3 (−rn5//−rp3) are controlled to besubstantially the same. As a result, the gain A6 can be increased.

In the sixth preferred embodiment, the substrate terminals of the NMOStransistors Mn3 and Mn4 are connected to the respective sources. Thesesubstrate terminals may alternatively be grounded, in which case theNMOS transistors Mn3 and Mn4 are subjected to substrate bias effect.This alternative configuration produces the same effect.

The foregoing description has been made with reference to the left halfof the circuit configuration. The right half of the circuit has the sameconfiguration as that of the left half, and produces the same effect.Further, even the absence of the constant current source It alsoprovides the same effect.

Seventh Preferred Embodiment

FIG. 11 is a circuit diagram showing the configuration of a differentialamplifier according to a seventh preferred embodiment of the presentinvention. In the seventh preferred embodiment, the differentialamplifier of the fifth preferred embodiment is more specificallyconfigured using MOS transistors opposite in polarity to those of thesixth preferred embodiment. Except for the presence of the circuits 13and 15, the differential amplifier of the seventh preferred embodimenthas the same configuration as that of the fourth preferred embodiment(FIG. 7). The constituents operating in the same manner as those of thefourth preferred embodiment are designated by the same referencenumerals, and will not be discussed.

The configuration of the circuit 13 will be discussed first. The NMOStransistor Mn3 has a drain connected to the drain of the PMOS transistorMp3, and a substrate terminal connected to the source that is grounded.

The configuration of the circuit 15 will be discussed next. The NMOStransistor Mn4 has a drain connected to the drain of the PMOS transistorMp4, and a substrate terminal connected to the source that is grounded.The prescribed voltage Vb3 is applied to the NMOS transistors Mn3 andMn4 such that the NMOS transistors Mn3 and Mn4 operate in a saturationregion.

Next, it will be discussed how the differential amplifier having theforegoing configuration operates. In the seventh preferred embodiment,the PMOS transistors Mp3, Mp5 and NMOS transistor Mn3 form the negativeresistance generation block 12. The PMOS transistor Mp5 corresponds tothe MOS transistor M1 (FIG. 9). The PMOS transistor Mp3 corresponds tothe constant current source I21 and the resistor Rne1 connected inparallel to the constant current source I21 (FIG. 9). The voltage Vb1 isapplied to the PMOS transistor Mp3 such that the PMOS transistor Mp3operates in a saturation region.

The NMOS transistor Mn3 corresponds to the constant current source I31and the resistor R31 connected in parallel to the constant currentsource I31 (FIG. 9). The voltage Vb3 is applied to the NMOS transistorMn3 such that the NMOS transistor Mn3 operates in a saturation region.Thus the output resistance rp3 of the PMOS transistor Mp3 corresponds tothe resistance Rne1, and an output resistance m3 of the NMOS transistorMn3 corresponds to the resistance R13. Accordingly, the negativeresistance generation block 12 generates a negative resistance(−rn3//−rp3).

The NMOS transistor Mn1 and PMOS transistor Mp1 respectively correspondto the load R1 and voltage-controlled current source T1. Thenidentifying the respective output resistances of the PMOS transistor Mp1and NMOS transistor Mn1 as rp1 and rn1, a gain A7 of the differentialamplifier of the seventh preferred embodiment is given by:A 7=−gmp 1·(rn 1//rp 1//−rn 3//−rp 3)

In the configuration discussed so far, the combined resistance of therespective output resistances rn1 and rp1 of the NMOS transistor Mn1 andPMOS transistor Mp1 (rn1//rp1), and the value of the combined resistanceof the respective output resistances rn3 and rp3 of the NMOS transistorMn3 and PMOS transistor Mp3 (−rn3//−rp3) are controlled to besubstantially the same. As a result, the gain A7 can be increased. Inthe seventh preferred embodiment, the substrate terminals of the PMOStransistors Mp5 and Mp6 are connected to the respective sources. Thesesubstrate terminals may alternatively be connected to the power sourcevoltage VDD, in which case the same effect can be obtained.

The differential amplifiers of the second preferred embodiment (FIG. 3)and fifth preferred embodiment (FIG. 9) may include cascode connection.By way of example, the circuit configuration of the differentialamplifier of the fifth preferred embodiment (FIG. 9) may include cascodeconnection as shown in FIG. 12. Here, cascode connection meansconnection between transistors connected in series such as the PMOS Mp1and Mp3 shown in FIG. 12.

The PMOS transistor Mp1 has a gate connected to the voltage source Vin1for receiving the input voltage Vin1, and a source connected through theconstant current source It to the power source voltage VDD.

The PMOS transistor Mp1 has a drain connected to the source of the PMOStransistor Mp3. The PMOS transistor Mp3 has a drain connected to theoutput terminal O1, and a gate subjected to the application of theprescribed voltage Vb1 such that the MOS transistor Mp3 operates in asaturation region. The PMOS transistors Mp1 and Mp3 form cascodeconnection.

The NMOS transistor Mn1 has as drain connected to the drain of the PMOStransistor Mp3 and to the output terminal O1, and a source connected tothe drain of the NMOS transistor Mn3. The source of the NMOS transistorMn3 is grounded. The prescribed voltages Vb2 and Vb3 are respectivelyapplied to the gates of the NMOS transistors Mn1 and Mn3 such that theNMOS transistors Mn1 and Mn3 operate in a saturation region. Thesubstrate terminals of the NMOS transistors Mn1 and Mn3 are grounded.The NMOS transistors Mn1 and Mn3 form cascode connection.

The PMOS transistor Mp2 has a source connected to the constant currentsource It, a drain connected to the source of the PMOS transistor Mp4,and a gate connected to the voltage source Vin2 for receiving the inputvoltage Vin2. The substrate terminal of the PMOS transistor Mp2 isconnected to the source.

The PMOS transistor Mp4 has a drain connected to the output terminal O2,and a substrate terminal connected to the power source voltage VDD. Theprescribed voltage Vb1 is applied to the gate of the PMOS transistor Mp4such that the PMOS transistor Mp4 operates in a saturation region. ThePMOS transistors Mp2 and Mp4 form cascode connection.

The NMOS transistor Mn2 has a drain connected to the drain of the PMOStransistor Mp4 and to the output terminal O2, and a source connected tothe drain of the NMOS transistor Mn4. The source of the NMOS transistorMn4 is grounded. The prescribed voltages Vb2 and Vb3 are respectivelyapplied to the gates of the NMOS transistors Mn2 and Mn4 such that theNMOS transistors Mn2 and Mn4 operate in a saturation region. Thesubstrate terminals of the NMOS transistors Mn2 and Mn4 are grounded.The NMOS transistors Mn2 and Mn4 form cascode connection.

The configuration of the negative resistance generation block 12 will bediscussed next. The PMOS transistor Mp5 has a source connected to thepower source voltage VDD, and a drain connected to the source of thePMOS transistor Mp7. The substrate terminals of the PMOS transistors Mp5and Mp7 are connected to the power source voltage VDD. Voltages Vb4 andVb5 are respectively applied to the gates of the PMOS transistors Mp5and Mp7 such that the PMOS transistors Mp5 and Mp7 operate in asaturation region. The PMOS transistors Mp5 and Mp7 form cascodeconnection.

A PMOS transistor Mp9 has a source connected to the drain of the PMOStransistor Mp7, a drain connected to the output terminal O1, a substrateterminal connected to the source, and a gate connected to the outputterminal O2.

The source of the PMOS transistor Mp9 is also connected to the drain ofthe NMOS transistor Mn5. The NMOS transistor Mn5 has a source connectedto the drain of the NMOS transistor Mn7. The source of the NMOStransistor Mn7 is grounded. The substrate terminals of the NMOStransistors Mn5 and Mn7 are grounded.

Voltages Vb6 and Vb7 are respectively applied to the gates of the NMOStransistors Mn5 and Mn7 such that the NMOS transistors Mn5 and Mn7operate in a saturation region. The NMOS transistors Mn5 and Mn7 formcascode connection.

The PMOS transistor Mp6 has a source connected to the power sourcevoltage VDD, and a drain connected to the source of a PMOS transistorMp8. The substrate terminals of the PMOS transistors Mp6 and Mp8 areconnected to the power source voltage VDD. The voltages Vb4 and Vb5 arerespectively applied to the gates of the PMOS transistors Mp6 and Mp8such that the PMOS transistors Mp6 and Mp8 operate in a saturationregion. The PMOS transistors Mp6 and Mp8 form cascode connection.

The drain of the PMOS transistor Mp8 is connected to the source of aPMOS transistor Mp10. The PMOS transistor Mp10 has a drain connected tothe output terminal O2, a substrate terminal connected to the source,and a gate connected to the output terminal O1.

The source of the PMOS transistor Mp10 is also connected to the drain ofthe NMOS transistor Mn6. The NMOS transistor Mn6 has a source connectedto the drain of the NMOS transistor Mn8. The source of the NMOStransistor Mn8 is grounded. The substrate terminals of the NMOStransistors Mn6 and Mn8 are grounded. The voltages Vb6 and Vb7 arerespectively applied to the gates of the NMOS transistors Mn6 and Mn8such that the NMOS transistors Mn6 and Mn8 operate in a saturationregion. The NMOS transistors Mn6 and Mn8 form cascode connection.

Next, it will be discussed how the differential amplifier having theforegoing configuration operates. In comparison with FIG. 9, the cascodeconnection of the NMOS transistors Mn1 and Mn3 corresponds to the loadR1. The output resistance R11 is given by ((gmn1·rn1)·rn3). The cascodeconnection of the PMOS transistors Mp1 and Mp3 corresponds to thevoltage-controlled current source T1. The output resistance Rin1 isgiven by ((gmp3·rp3)·rp1).

The output resistance of the cascode connection formed by the PMOStransistors Mp5 and Mp7 corresponds to the resistance of the resistorRne1 and is given by ((gmp7·rp7)·rp5). The cascode connection of theNMOS transistors Mn5 and Mn7 corresponds to the circuit 13. The outputresistance R13 is given by ((gmn5·rn5)·rn7).

As a result of the foregoing correspondences, a gain A8 of thedifferential amplifier shown in FIG. 12 is given by:A 8=gmp 1·((gmn 1·rn 1)rn 3//(gmp 3·rp 3) rp 1//−(gmn 5·rn 5)rn 7//−(gmp7·rp 7)rp 5)where gmp1, gmp3 and gmp7 represent respective mutual conductances ofthe PMOS transistors Mp1, Mp3 and Mp7, and gmn1 and gmn5 representrespective mutual conductance of the NMOS transistors Mn1 and Mn5.Further, m1, m5 and m7 represent respective output resistances of theNMOS transistors Mn1, Mn5 and Mn7, and rp1, rp3, rp5 and rp7 representrespective output resistances of the PMOS transistors Mp1, Mp3, Mp5 andMp7.

In the configuration discussed so far in which the differentialamplifier includes cascode connections, the output resistance of thenegative resistance generation block 12 and that of the configurationexcluding the negative resistance generation block 12 are controlled tobe substantially the same, to thereby realize increase of a gain.

While the invention has been shown and described in detail, theforegoing description is in all aspects illustrative and notrestrictive. It is therefore understood that numerous modifications andvariations can be devised without departing from the scope of theinvention.

1. A current source circuit, comprising: an output terminal; a currentsource including an internal resistor and connected to said outputterminal; and a negative resistance generation block generating anegative resistance and connected in parallel to said current source. 2.The current source circuit according to claim 1, wherein said negativeresistance generation block comprises: a voltage-controlled currentsource having a current input terminal connected to said outputterminal, and a voltage-controlled terminal receiving a signal invertedwith respect to an output signal sent from said output terminal; and asecond current source including an internal resistor having one endconnected to a current output terminal of said voltage-controlledcurrent source, and another end that is grounded.
 3. The current sourcecircuit according to claim 2, wherein said voltage-controlled currentsource is an MOS transistor, and wherein said current source and saidsecond current source are MOS transistors operating in a saturationregion.
 4. A current source circuit, comprising: an output terminal; afirst current source including an internal resistor and connected tosaid output terminal; an inverting amplifier for inverting an outputsignal sent from said output terminal to provide an inverted outputsignal; an MOS transistor having a current input terminal connected tosaid output terminal, and a voltage-controlled terminal connected to theoutput of said inverting amplifier and receiving said inverted outputsignal; and a second current source including an internal resistorhaving one end connected to a current output terminal of said MOStransistor, and another end that is grounded.
 5. A differentialamplifier, comprising: a differential input terminal; a differentialoutput terminal providing a differential output signal formed byamplifying a differential input signal sent from said differential inputterminal; and a negative resistance generation block generating anegative resistance and connected to said differential output terminal.6. The differential amplifier according to claim 5, further comprising asecond differential output terminal providing an inverted differentialoutput signal inverted with respect to said differential output signal,wherein said negative resistance generation block comprises: avoltage-controlled current source having a current input terminalconnected to said differential output terminal, and a voltage-controlledterminal connected to said second differential output terminal andreceiving said inverted differential output signal; and a current sourceincluding an internal resistor having one end connected to a currentoutput terminal of said voltage-controlled current source, and anotherend that is grounded.
 7. The differential amplifier according to claim6, wherein said voltage-controlled current source is an MOS transistor,and wherein said current source is an MOS transistor operating in asaturation region.
 8. The differential amplifier according to claim 6,wherein said voltage-controlled current source is an MOS transistor, andwherein said current source includes cascode connection of transistors.9. A differential amplifier, comprising: a differential input terminal;a first differential output terminal providing a differential outputsignal formed by amplifying a differential input signal sent from saiddifferential input terminal; a second differential output terminalproviding an inverted differential output signal inverted with respectto said differential output signal; a load having one end connected to apower source voltage and another end connected to said firstdifferential output terminal; a voltage-controlled current source havinga current input terminal connected to said first differential outputterminal, and a voltage-controlled terminal connected to saiddifferential input terminal and receiving said differential inputsignal; a constant current source having one end connected to a currentoutput terminal of said voltage-controlled current source, and anotherend that is grounded; an MOS transistor having a current input terminalconnected to said first differential output terminal, and avoltage-controlled terminal connected to said second differential outputterminal and receiving said inverted differential output signal; and acurrent source including an internal resistor having one end connectedto a current output terminal of said MOS transistor, and another endthat is grounded.
 10. The differential amplifier according to claim 6,wherein said negative resistance generation block further comprises asecond current source ac-coupled in parallel to said current source. 11.The differential amplifier according to claim 10, wherein saidvoltage-controlled current source is an MOS transistor, and wherein saidcurrent source and said second current source are MOS transistorsoperating in a saturation region.
 12. The differential amplifieraccording to claim 10, wherein said voltage-controlled current source isan MOS transistor, and wherein said current source and said secondcurrent source include cascode connection of transistors.
 13. Adifferential amplifier, comprising: a differential input terminal; afirst differential output terminal providing a differential outputsignal formed by amplifying a differential input signal sent from saiddifferential input terminal; a second differential output terminalproviding an inverted differential output signal inverted with respectto said differential output signal; a load having one end connected to apower source voltage and another end connected to said firstdifferential output terminal; a voltage-controlled current source havinga current input terminal connected to said first differential outputterminal, and a voltage-controlled terminal connected to saiddifferential input terminal and receiving said differential inputsignal; a constant current source having one end connected to a currentoutput terminal of said voltage-controlled current source, and anotherend that is grounded; an MOS transistor having a current input terminalconnected to said first differential output terminal, and avoltage-controlled terminal connected to said second differential outputterminal and receiving said inverted differential output signal; a firstcurrent source including an internal resistor having one end connectedto a current output terminal of said MOS transistor, and another endthat is grounded; and a second current source having one end connectedto said power source voltage and another end connected to said firstcurrent source.